The present invention generally relates to metal-surface-treating agents and surface-treated metal materials treated using the same. More particularly, the present invention relates to a surface-treated metal material having a superior conductivity and corrosion resistance and to a metal-surface treating composition of a chromium-free surface-treating agent capable of making such a metal.
Conventionally, zinc-type plated steel sheets such as galvanized steel sheets and zinc-aluminum plated steel sheets have been widely used in the fields of home appliances, automotive vehicles, and construction. In order to improve the corrosion resistance of these steel sheets, chromate films and organic films on the chromate films may be provided on the plated surfaces thereof. When the organic films are provided, the chromate films serve to improve the adhesion to the organic films.
While the chromate films have superior corrosion resistance and adhesiveness to coatings, a special effluent treatment prescribed by the Clean Water Act is required during the chromate coating step, because the films contain hexavalent chromium, resulting in higher costs.
Accordingly, a chromium-free surface-treating technique has been desired in order to prevent generation of white rust on steel sheets, particularly in the zinc-type-plated steel sheets. There are a number of techniques suggested as exemplified below.
1. In Japanese Unexamined Patent Application Publication No. 5-195244, a metal-surface-treating method comprising a chromium-free composition, containing (a) an anion component comprising at least four fluorine atoms and at least one atom of titanium, zirconium, or the like (for example, titanium fluoride represented by (TiF62xe2x88x92)), (b) a cation component such as cobalt or magnesium, (c) a free acid for pH adjustment, and (d) an organic resin, is proposed. However, according to the description, the surface-treated metal sheets obtained by this method exhibit corrosion resistance only when conventional protective compositions for undercoating and finish-coating are applied on the surfaces thereof. The resulting film alone does not provide a sufficient corrosion resistance.
2. In Japanese Unexamined Patent Application Publication No. 9-241856, a metal-surface-treating method comprising a chromium-free composition containing (a) a hydroxylic copolymer, (b) phosphorous, and (c) a phosphate of metal such as copper or cobalt is suggested. Although the surface-treated metal sheets obtained by this method exhibit a superior bare corrosion resistance in an as-worked state and a satisfactory adhesiveness to coatings, it is difficult to obtain conductivity due to a dense film formed by the crosslinks between various metal phosphates and the resin.
3. In Japanese Unexamined Patent Application Publication No. 11-50010, a metal-surface-treating agent comprising a chromium-free composition containing (a) a resin having a copolymer segment comprising a segment of polyhydroxyether and an unsaturated monomer, (b) phosphoric acid, and (c) a phosphate of metal such as calcium or cobalt is suggested. However, while the surface-treated metal sheets obtained by this method exhibit superior bare corrosion resistance, it is difficult to ensure conductivity due to a dense film formed by the crosslinks between various metallic phosphates and the resin.
4. In Japanese Unexamined Patent Application Publication No. 11-1069450, an aqueous surface-treating agent prepared by dissolving (a) ions of polyvalent metal such as manganese and cobalt, (b) an acid such as fluoro acid or phosphoric acid, (c) a silane coupling agent, and (d) an aqueous polymer having a polymerization unit of 2 to 50 in an aqueous solvent is proposed. However, in order to guarantee corrosion resistance, insoluble resin films formed by the etching effect of the acid composition contained in the treating agent are provided on the surfaces of the surface-treated metal sheets obtained by using this surface-treating agent. This film is mainly composed of a resin component, and it is therefore difficult to obtain conductivity.
5. In Japanese Unexamined Patent Application Publication No. 11-29724, a method for providing a coating on zinc-coated steel by using an aqueous rust-proof coating agent containing (a) a thiocarbonylic compound, (b) phosphate ions, (c) a water-dispersible silica is proposed. Sulfides such as thiocarbonyl compounds easily adhere to the surfaces of metal such as zinc; when sulfides are provided together with phosphate ions, thiol ions in the thiocarbonyl compound adhere to the active sites on the surface of zinc during coating, thereby exhibiting rust-resistance. However, while the resulting zinc-coated steel or non-coated steel exhibits satisfactory anti-corrosion properties when the surfaces thereof are coated with layers containing xe2x95x90Nxe2x80x94C(xe2x95x90S)xe2x80x94 groups or xe2x80x94Oxe2x80x94C(xe2x95x90S)xe2x80x94 groups, the steel exhibits no conductivity. Moreover, when the thickness of the coat is reduced so as to obtain conductivity, some portions are not coated by the thiocarbonyl compound, causing rust to develop. In short, anti-corrosion properties and conductivity are not compatible with each other even in this method.
6. In Japanese Unexamined Patent Application Publication No. 9-31665, a method for surface-treating magnetic steel sheets by using a chromium-free composition including (a) a copolymer containing hydroxyl groups and carboxyl groups, (b) phosphoric acid, (c) at least one of Mn, Al, Ba, Ca, and Sr heavy metal compounds, (d) a heavy metal containing at least one of SiO2, ZrO2, SnO2, Al2O3, and Sb2O5 colloids, is suggested. While the resulting magnetic steel sheets exhibit satisfactory corrosion resistance, it is difficult to obtain conductivity due to a dense film formed of monomers containing hydroxyl groups and monomers containing carboxyl groups.
In the above-described methods 1 to 4, an acceptable corrosion resistance is achieved when the metal sheets are coated using sufficient amounts of the surface treating agent (coating agent), i.e., when the thickness of the coat is sufficient. However, when a partial coating is performed, leaving an exposed portion, for example, a projecting portion, on the metal sheet or when the thickness of the coat is significantly reduced, corrosion resistance thereof is drastically degraded. In other words, a sufficient corrosion resistance is exhibited only when the metal sheet is covered with the surface-treating agent at a ratio of 100%, but not when the ratio is less than 100%. Among these surface-treating agents, those in methods 2 to 4 obtain corrosion resistance due to the configuration of the resin coat yielded by the crosslinks between the metallic salt and resin. Therefore, an increase in the thickness over the entire coat results in the degradation of the conductivity. When the thickness of the coat is reduced so as to improve the conductivity, the corrosion resistance is degraded.
Moreover, the above-described conventional arts 1 to 6 aim to achieve a firm adhesion at the interface between the metal surface and the coat formed by the surface-treating agent. From a microscopic point of view, the metal surface and the surface-treating agent do not completely adhere to each other and there is a limit as to the possible improvements on the adhesive property. Accordingly, in these conventional techniques, the improvements on the density of the coat provided by the surface-treating agent has been the foremost important object over improvements on the adhesive property. This object is not compatible with the improvements of conductivity.
Recently, there is an increasing demand for chromium-free surface-treated metal sheets exhibiting corrosion resistance and low surface resistance to be used in office appliances such as copying machines and home appliances such as air conditioners. The steel sheets having low surface resistance, i.e., the steel sheets having a high conductivity, prevent leakage of noise generated by electromagnetic waves. However, among a number of conventional suggestions made regarding the chromium-free art, no art discloses zinc-type-plated steel sheets exhibiting a satisfactory conductivity at the same time with corrosion resistance, or a surface-treating composition for making the same.
In view of the above, an object of the present invention is to provide a surface-treated metal material, typically a surface-treated zinc-type plated steel sheet, having superior corrosion resistance and conductivity, which does not require special effluent treatments when the material is worked and during the step of coating using the surface-treating agent.
To achieve the above object, the present inventor has conducted an extensive research to complete the present invention.
The present invention relates to a metal-surface-treating composition comprising metal ions of Al, Mg, and Mn, a water-soluble organic resin, an acid, and water.
Preferably, in the composition, the water-soluble organic resin comprises at least one selected from the group consisting of polymers of carboxylic monomers, and copolymers of carboxylic monomers and other polymerizable monomers.
Preferably, in each of the above-described compositions, the acid is at least one selected from the group consisting of phosphoric acid, acetic acid, nitric acid, and hydrofluoric acid. More preferably, in this case, the above-described composition further contains an organic acid capable of being coordinated with polyvalent metals.
Preferably, the composition contains at least one selected from the group consisting of Zn, Co, Ti, Sn, Ni, Fe, Zr, Sr, Y, Nb, Cu, Ca, V, and Ba ions.
Preferably, the composition further contains polyphenol carboxylic acids.
Preferably, the composition further contains a water-dispersible resin.
Preferably, the composition further contains a coupling agent.
The present invention also provides a surface-treated metal material having a coat formed by applying one of the above-described metal-surface-treating compositions on a metal material. Preferably, the surface-treated metal material uses a zinc-type plated steel sheet as the metal material.
In this application, when a coat is formed by applying the surface-treating composition of the present invention on the surface of the metal material, the corrosion resistance is not degraded even when the thickness of the coat is significantly reduced, and a surface-treated metal material having superior conductivity can be obtained. Moreover, because the surface-treating composition of the present invention is chromium free, water pollution by chromium does not occur.